1. Field of the Invention
The present invention relates, in general, to a method for the synthesis of diamond film utilizing high density direct current glow discharge and, more particularly, to a method for depositing a diamond film by high density direct current glow discharge employing a plurality of separate, space-distant U-shaped filaments as a cathode and keeping the temperature of the cathode high, thereby uniformly producing high quality diamond film on a large area of substrate.
2. Description of the Prior Art
Direct current (DC) glow discharge chemical vapor deposition (CVD), a vapor synthesis technique representative for diamond film synthesis, is well known to those skilled in the art.
In order to better understand the background of the present invention, a conventional method is described with reference to FIGS. 1 through 3.
Referring initially to FIG. 1, there is shown a DC glow discharge CVD apparatus for synthesizing a diamond film. As shown in this figure, the DC glow discharge CVD apparatus is composed of a reactor 1 having a cathode 2 supported by a cathode holder 7 through a cathode suspending bar 6 and an anode 3 opposite to the cathode 2 to a distance, a vacuum pump 4 to keep the pressure of the reactor low, a gas supplier 5 to introduce material gas into the reactor, and a DC power supplier 8 connected with the cathode holder 7. Using such DC glow discharge deposition apparatus, a diamond film is synthesized on a substrate 9. In detail, a voltage formed by the DC power supplier 8 generates plasma between the cathode 2 and the anode 3 which in turn decomposes the material gas introduced from the gas supplier 5.
When a diamond film is synthesized by use of the apparatus, in order to keep the deposition rate of the diamond film high, for example, up to several tens of .mu.m/h, the discharge must be carried out in a glow-arc transition range under such a condition that the pressure of the material gas is in a range of about 100 to about 200 torr and discharge current density is not less than 0.5 A/cm.sup.2 (see Japanese Patent Publication No. Heisei 2-133398).
During such high density glow discharge, there is apparently formed a luminous region over the substrate 9 atop the anode 3, which is called a positive column (PC) (see Kazuhiro Suzuki, Atsuhito Sawabe and Inuzuka, Jap. J. Appl. Phys., 29 (1990) 153).
Since the diamond film is coated only on an area of the substrate contacted with the positive column or on an area of the substrate adjacent to the positive column, the uniform thickness and properties of the diamond film deposited demand that the positive column be formed uniformly and symmetrically to the central axis of the substrate.
Meanwhile, in diamond deposition technology using vapor synthesis process, it is very industrially important that the diamond film is deposited as widely and thickly as possible as to have sufficient mechanical durability. Particularly, in order to use the diamond film formed by vapor synthesis process for an infrared optical window, a brazing-type cutting tool, a semiconductor substrate or a heat sink, it should be a free-standing film having a thickness of about 100 to about several hundred microns.
It takes about 50 hours to obtain a 1 mm thick diamond film even under a deposition rate of 20 .mu.m/h, the fastest rate of the DC glow discharge method reported thus far. For stable deposition for such a long time, it is required that the plasma not be extinguished and the discharge current be stably maintained without large deterioration. However, when a diamond film is synthesized by the conventional high density glow discharge, an irregular arc is suddenly generated between the cathode and the substrate, making the positive column be extinguished. Therefore, it is impossible in practice to stably deposit the diamond film for a long time with the conventional high density glow discharge.
Existing researchers have recognized that the extinction of the plasma by sudden generation of such arc is attributed to the electric properties of discharge and have made many attempts to cope with it in an aspect of electric circuits. For example, there were suggested use of a ballast resistance having a large capacity of around 1,000 ohms [K. Suzuki and T. Inuzuka, Surface Technology, 42 (1991) 1190, use of direct current pulse supplier in place of direct current power supplier [D. Satrapa, R. Haubner and B. Lux, Surface and Coatings Technology, 47 (1991) 59], and application of magnetic fields to both electrodes [N. Nesladek, "Investigation of rotating DC-discharge for diamond deposition", in Diamond 1992, Proc. Third Int. Conf. on The New Diamond Science and Tech. jointly with 3rd European Conf. Diamond, Diamond-like and Related Coatings, Aug. 31, Sep. 4, 1992, Heidelberg, Germany, Ed. by P. K. Bachmann, A. T. Collins, M. Seal, Elsevier Sequoia, (1993) 357].
However, there are pointed out many problems in their attempts. In detail, a ballast resistance having a large capacity consumes too large a quantity of power at the ballast resistance to deposit a diamond film on a large area of substrate at a high rate, in practice.
The direct current pulse supplier is disadvantageous in many aspects. First, a pulse generator for direct current pulse is high in cost as compared with ordinary direct current suppliers. Since the pulse generator supplies only a small quantity of direct current power per hour, its use causes the slow deposition rate. Moreover, there is no report that DC pulse generator is successfully employed to produce diamond film on a large substrate with a high growth rate.
With regard to the application of magnetic fields to both electrodes, it is troublesome in that a circuit for forming magnetic fields is additionally installed in the both electrodes. What is still worse, the application of magnetic fields to both electrodes can not prevent the generation of an arc completely, either.
Meanwhile, a diamond film deposition method using the conventional high density DC glow discharge has difficulty in forming a positive column uniformly on a large area of substrate having a diameter of 3 to 5 cm and thus, is almost incapable of depositing a diamond film which has uniform thickness and properties for a larger area of substrate. Neither can it synthesize a diamond film with a sufficient thickness.
When a diamond film is synthesized by the high density direct current discharge technology, it is recognized that the area of substrate deposited with the diamond film has close relations with the size and shape of the cathode. Such conventional direct current glow discharge technology mainly employs a cathode with the shape of a disk, rod or tube.
Now, referring to FIGS. 2A and 2B, there are illustrated plasma phases formed by the conventional high density direct current glow discharge technology employing a disk-shaped cathode which are dependent on the diameter of the cathode. FIG. 2A shows a small cathode with a diameter of about 1 cm, while FIG. 2B shows a large cathode with a diameter of 3 to 5 cm.
As shown in FIG. 2A, a positive column 13 formed between a cathode 12 supported by cathode support bar 11 and a substrate 10 is uniform and symmetrical to the central axis of the substrate 10. However, since the size of the positive column 13 formed is small, the area deposited is limited.
The diameter of the positive column is proportional to that of the cathode to some extent. Accordingly, it is expected that a large diameter of the cathode, for example, a 3 to 5 cm diameter, would make a large positive column proportional to the diameter. However, the corresponding area of the anode to the cathode 12' can not be uniformly heated but is only locally heated by the discharge current, in practice, so that the upper portion of the positive column 13' is biased toward a heat-concentrated region, as shown in FIG. 2B. Therefore, it is impossible to uniformly deposit a diamond film on a large area of substrate with a cathod of large diameter. This trend is aggravated as the diameter of the cathode becomes larger.
For deposition of diamond film on large areas, it is required that the area of the cathode be augmented proportionally to that of the substrate, which casts a serious restriction on the subject.
To prevent the bias phenomenon of the plasma, the cathode should be uniformly heated. For this, it is preferred to keep the specific surface area of the cathode as large as possible.
A representative shape having the largest specific surface area yet maintaining its durability is a filament shape. For example, spiral filament is widely used in various electrical heaters and for diamond vapor deposition technique, such as hot filament technique or EVCVD technique.
Referring to FIG. 3, there is shown a spiral filament which is employed as a cathode in a high density direct current glow discharge apparatus. FIG. 3A shows an early stage of discharge whereas FIG. 3B shows a later stage of discharge. FIGS. 3C and 3D each illustrates a supporting structure of filament.
As shown in FIG. 3A, a spiral filament 14 with a diameter of about 20 to about 30 mm is newly mounted and, within several hours, a barrel-shaped positive column 16 is formed on a large area of a substrate 15 having a diameter of about 30 to about 50 mm symmetrically to the central axis of the substrate 15. Thus, a uniform deposition is accomplished by the spiral filament if its operation is finished within several hours.
However, with the lapse of time, the spiral filament is twisted seriously. After some time, as shown in FIG. 3B, the twisted filament 14' makes a positive column 16' biased toward one side, preventing a diamond film from being deposited uniformly.
Now, a description will be made in conjunction with the reason for the biased positive column.
The larger the diameter of spiral filament is, the more serious is temperature difference between a lower portion of the filament, the portion being in contact with the positive column and an upper portion of the filament, the portion being distant therefrom. For example, even when the lower portion is heated to above 2,100.degree. C., the upper portion has a temperature of about 1,000.degree. to about 1,300.degree. C. Thus, there is an extreme temperature difference ranging from about 800.degree. to about 1,100.degree. C. therebetween.
Generally, a spiral filament is made of a metal having a high melting point, such as tungsten, tantalum and the like. It has been reported that such filament is carburized in a high density of a diamond film synthesizing atmosphere at a temperature of 2,100.degree. to 2,300.degree. C. At this time, the nature of carburization of the filament is different according to the local temperature. In other words, there are scarcely observed cracks or splits in the lower portion of filament heated to above 2,100.degree. C., whereas many cracks and splits are generated in the upper portion of filament having a temperature lower by several hundred Celsius degrees than that of the lower portion.
Since the conventional spiral filament is structured to be a single body of one conductive wire, even if no deformation occurs in the lower portion during the carburization, any deformation at a region of the upper portion causes the total filament to twist. As a result, the positive becomes biased, for example, like FIG. 3B.